What can cause dark blood and blood clots in stool?

Bleeding.: Bleeding in your GI tract. This needs evaluation by your doc soon if not immediately.

Many possibilities: Hemorrhoid, anal fissure, anal fistula, draining peri-rectal abscess, colitis, colon polyps, bleeding stomach ulcers, bleeding colon diverticula, colon vascular malformation, colon or rectal cancer, and maybe others. Need to see a doctor for an examination!